If you are already working out with weights, and do not just want to become stronger and more muscular but also fitter, you can combine strength training with interval training. Adding interval training to you training routines will not cost you muscle mass. At most it will slightly diminish the increase in muscle strength in your legs. And according to Australian sports scientists, there are even some things you can do about that.

Study
The researchers, who are affiliated with the University of Sydney, traced 13 studies in the scientific literature, in which fellow scientists compared the effect of strength training with that of strength training plus interval training. Details about the schedules used can be found in the unreadable table below. If you click on it, a larger version will pop up.

The researchers aggregated the research results and analyzed them again.

Results
Adding interval training to strength training had no negative influence on the increase in muscle mass, nor on the increase in strength in the upper body. However, there was a negative effect on the increase in strength in the lower body.

That negative effect was small. [Table] According to the meta-analysis, the maximum lower body strength in the subjects who only did strength training increased by 23.9 percent. In addition, when the test subjects also took interval training, their maximum lower body strength increased by 19.4 percent.

Ladies and gentlemen, what are we talking about?

When the researchers looked closely at their data, they discovered two ways in which athletes can reduce the small effect of interval training on the increase in strength. They summarize those below.

Conclusion
"The findings of this review show that high intensity interval training can be prescribed alongside resistance training without negatively impacting changes in lean muscle mass and that any attenuation of lower body muscular strength might be ameliorated by prescribing running based high intensity interval training and providing adequate rest between high intensity interval training and resistance training sessions", write the researchers.

"It is important to consider that concurrent high intensity interval training and resistance training as well as resistance training alone both improved dynamic strength across all studies and that the difference in lower body strength between the two groups was only small."

"Also, because high intensity interval training has been shown to improve VO2max, sprint time and maximal aerobic speed, the slight reduction in [the increase of] lower body strength may be a small price to pay for improvements in other key aspects of sporting performance particularly under time constraints."